We report the first detection of X-ray emission in a pre-planetary nebula,
Hen 3-1475. Pre-planetary nebulae are rare objects in the short transition
stage between the Asymptotic Giant Branch and planetary nebula evolutionary
phases, and Hen 3-1475, characterised by a remarkable S-shaped chain of optical
knots, is one of the most noteworthy members of this class. Observations with
the Advanced CCD Imaging Spectrometer (ACIS) onboard the Chandra X-Ray
observatory show the presence of compact emission coincident with the brightest
optical knot in this bipolar object, which is displaced from the central star
by 2.7 arcsec along the polar axis. Model fits to the X-ray spectrum indicate
an X-ray temperature and luminosity, respectively, of (4.3-5.7) 10^6 K and
(4+/-1.4) 10^{31} (D/5 kpc)^2 erg s^{-1}, respectively. Our 3-sigma upper limit
on the luminosity of compact X-ray emission from the central star in Hen 3-1475
is ~5 10^{31} (D/5 kpc)^2 erg s^{-1}. The detection of X-rays in Hen 3-1475 is
consistent with models in which fast collimated post-AGB outflows are crucial
to the shaping of planetary nebulae; we discuss such models in the context of
our observations.Comment: 2 figure

Blackman, Eric G.

Frank, Adam

Kastner, Joel H.

Morris, Mark

Sahai, Raghvendra

English

arXiv

We report the first detection of X-ray emission in a pre-planetary nebula, Hen 3-1475. Pre-planetary nebulae are rare objects in the short transition stage between the Asymptotic Giant Branch and planetary nebula evolutionary phases, and Hen 3-1475, characterized by a remarkable S-shaped chain of optical knots, is one of the most noteworthy members of this class. Observations with the Advanced CCD Imaging Spectrometer (ACIS) onboard the Chandra X-Ray observatory show the presence of compact emission coincident with the brightest optical knot in this bipolar object, which is displaced from the central star by 2.7  along the polar axis. Model fits to the X-ray spectrum indicate an X-ray temperature and luminosity, respectively, of (4.3 − 5.7)×10^6 K and (4 ± 1.4) × 10^31 (D/5 kpc)^2 erg s^−1, respectively. Our 3-sigma upper limit on the luminosity of compact X-ray emission from the central star in Hen 3-1475 is ~5 × 10^31 (D/5 kpc)^2 erg s^−1. The detection of X-rays in Hen 3-1475 is consistent with models in which fast collimated post-AGB outflows are crucial to the shaping of planetary nebulae; we discuss such models in the context of our observations (Refer to PDF file for exact formulas)

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Rochester Institute of TechnologyRIT Scholar WorksArticles12-20-2003X-ray emission from the pre-planetary nebulaHenize 3-1475Raghvendra SahaiJoel KastnerAdam FrankFollow this and additional works at: http://scholarworks.rit.edu/articleThis Article is brought to you for free and open access by RIT Scholar Works. It has been accepted for inclusion in Articles by an authorizedadministrator of RIT Scholar Works. For more information, please contact ritscholarworks@rit.edu.Recommended CitationAstrophys.J.L. 599 (2003) L87-L90L87The Astrophysical Journal, 599:L87–L90, 2003 December 20 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.X-RAY EMISSION FROM THE PRE–PLANETARY NEBULA HENIZE 3-1475Raghvendra Sahai,1 Joel H. Kastner,2 Adam Frank,3 Mark Morris,4 and Eric G. Blackman3Received 2003 September 16; accepted 2003 November 12; published 2003 December 4ABSTRACTWe report the first detection of X-ray emission in a pre–planetary nebula, He 3-1475. Pre–planetary nebulaeare rare objects in the short transition stage between the asymptotic giant branch (AGB) and planetary nebulaevolutionary phases, and He 3-1475, characterized by a remarkable S-shaped chain of optical knots, is one ofthe most noteworthy members of this class. Observations with the Advanced CCD Imaging Spectrometer onboard the Chandra X-Ray Observatory show the presence of compact emission coincident with the brightestoptical knot in this bipolar object, which is displaced from the central star by 2.7 along the polar axis. Modelfits to the X-ray spectrum indicate an X-ray temperature and luminosity, respectively, of K and6(4.3–5.7)# 10(D/5 kpc)2 ergs s1, respectively. Our 3 j upper limit on the luminosity of compact X-ray31(4 1.4)# 10emission from the central star in He 3-1475 is ∼ (D/5 kpc)2 ergs s1. The detection of X-rays in He 3-315# 101475 is consistent with models in which fast collimated post-AGB outflows are crucial to the shaping of planetarynebulae; we discuss such models in the context of our observations.Subject headings: circumstellar matter — ISM: jets and outflows —planetary nebulae: individual (He 3-1475) — stars: AGB and post-AGB —stars: mass loss — X-rays: ISM1. INTRODUCTIONPre–planetary nebulae (pre-PNs)5—objects in transition be-tween the asymptotic giant branch (AGB) and PN phases—hold the key to some of the most vexing problems in ourunderstanding of these very late stages of stellar evolution forintermediate-mass (∼1–8 M,) stars. Observationally, PNsshow a dazzling variety of morphologies (e.g., Schwarz, Cor-radi, & Melnick 1992; Sahai & Trauger 1998), whereas AGBstars are surrounded by roughly spherical gas-dust envelopesdue to dense, slow stellar winds ejected with rates up to 104M, yr1 (e.g., Neri et al. 1998). In the short transition periodbetween the AGB and PN phases, these objects turn on veryfast winds (with speeds of a few# 100 to ∼2000 km s1),which interact violently with the surrounding envelopes anddrastically modify their spatio-kinematic structures. The natureand origin of such outflows remains unknown, and their in-teraction with the ambient circumstellar material and the con-sequent changes in the kinematics and morphology of the cir-cumstellar material are very poorly understood.X-ray (0.1–10 keV) observations can probe the direct sig-nature of the interaction of the fast and slow winds, i.e., thevery hot (107–108 K) gas that should be produced in the shockedregion. As a result, many searches for X-ray emission fromPNs have been conducted beginning with the Einstein Obser-vatory (mid-1980s), followed by EXOSAT and ROSAT, andcontinue now with the Chandra X-Ray Observatory (CXO) andXMM-Newton. Prior to ROSAT, most X-rays in PNs were in-terpreted as coming from the hot [(1–2)# 105 K] central star;1 Jet Propulsion Laboratory, MS 183-900, California Institute of Technology,Pasadena, CA 91109; raghvendra.sahai@jpl.nasa.gov.2 Chester F. Carlson Center for Imaging Science, Rochester Institute ofTechnology, 54 Lomb Memorial Drive, Rochester, NY 14623.3 Department of Physics and Astronomy, University of Rochester, Rochester,NY 14627.4 Division of Astronomy and Astrophysics, University of California at LosAngeles, 405 Hilgard Avenue, Los Angeles, CA 90095.5 We prefer this phrase to “proto–planetary” nebula, which although in morecommon use for these objects is also potentially misleading, because a similarterm (protoplanetary) is used by the young stellar object community.ROSAT data led to the identification of diffuse emission fromseven PNs, but several of these were erroneous (Kreysing etal. 1992; Guerrero, Chu, & Gruendl 2000).With CXO and XMM, extended X-ray emission from PNshas been unambiguously imaged, and results have been re-ported for five objects (BD303639, NGC 7027, Mz 3: Kast-ner et al. 2000, 2001, 2003; NGC 6543: Chu et al. 2001; NGC7009: Guerrero, Gruendl, & Chu 2002). These studies clearlyshow the presence of extended bubbles of hot (∼106 K) gas,in qualitative agreement with our expectations from interacting-winds models.X-ray observations of pre-PNs provide us with a unique op-portunity to study magnetic fields, which may play an importantrole in shaping PNs (e.g., Balick & Frank 2002), indirectly (act-ing as the collimation agent of the fast outflows; Garcia-Segura1997; Gardiner & Frank 2001) or directly (magneto-centrifugallaunching of disk winds; Blackman, Frank, & Welch 2001).Unlike most PNs, the central stars of pre-PNs are too cold toprovide blackbody X-ray emission; thus, detection of a compactcentral source of X-rays in a pre-PN could provide direct ev-idence for the presence of a strong stellar magnetic field. Forexample, by analogy with the solar dynamo, new AGB stellardynamo modeling (Blackman et al. 2001a) predicts a strongfield (of a few# 104 G) near the base of the star’s convectionzone with a 0.2 yr decay half-cycle (11 yr for the Sun); dis-sipation of this field should result in a nonthermal X-ray lu-minosity of ∼1032 ergs s1 from the central stars in very youngpre-PNs.However, no systematic search for X-ray emission from pre-PNs has been carried out to date, because until very recently, itwas believed that the wind-wind interaction responsible for shap-ing PNs occurred during the PN phase (through the interactionof a fast radiatively driven spherical wind from the central hotwhite dwarf with an equatorially dense wind from the AGBphase). In this Letter, we report our discovery of X-ray emissionfrom the well-studied pre-PN He 3-1475. This remarkable S-shaped object was selected as the prime target in our search forX-ray emission from pre-PNs because it shows the presence ofan ultrafast outflow (∼2300 km s1; Sa´nchez Contreras & SahaiL88 SAHAI ET AL. Vol. 599Fig. 1.—CXO/ACIS-S3 X-ray image (blue, linear stretch) overlaid on acomposite HST image of He 3-1475 taken through the F507N ([O iii], green,log stretch) and F658N ([N ii], red, log stretch) emission-line filters.Fig. 2.—ACIS-S3 (background-subtracted) spectrum of He 3-1475, togetherwith a best-fit VMEKAL plasma model with K.6T p 5# 10X2001) and bright optical knots of emission from shocked gas(Borkowski & Harrington 2001; Riera et al. 2003).2. OBSERVATIONS AND RESULTSWe observed He 3-1475 with the CXO for 49.7 ks on 2002July 15, using the back-illuminated CCD S3 of the AdvancedCCD Imaging Spectrometer (ACIS). The CXO/ACIS combi-nation is sensitive over the energy range 0.3–10 keV. ACIShas a pixel size of 0.49, very similar to the width of the point-spread function (PSF) of the Chandra mirrors. The data, con-sisting of individual CCD X-ray and particle events, were sub-ject to standard processing by Chandra X-ray Center pipelinesoftware (CIAO,6 ver. 2.0), which determines the distributionof photon-generated charge within a 3# 3 CCD pixel boxcentered on the event position, flags events likely due to par-ticles, and computes the celestial positions and nominal ener-gies of incident X-rays. In order to optimize image resolution,the event charge distributions were used, along with the tele-scope pointing history and nominal photon energies, to cal-culate a subpixel position for each X-ray (Li et al. 2003).The CXO/ACIS-S3 image of He 3-1475 (Fig. 1) shows thepresence of compact emission coincident with the brightestoptical knot in this object (NW1a in Fig. 1 of Riera et al. 2003),as seen in a Hubble Space Telescope (HST) image taken withthe Wide Field Planetary Camera 2 in the [O iii] (F502N) filter.The relative registration of the optical and X-ray images hasbeen established as follows: (1) the USNO-B.1 catalog wasused to determine the optical position of the central star in He3-1475 (ap 17h45m14s.186, dp 175646.23 [J2000.0]),and (2) the absolute CXO astrometry was used for the X-raydata, which locates the central peak of the X-ray emission atap 17h45m14s.05, dp 175644.81, i.e., offset by 2.0 west,1.4 north from the central star. The peak of the optical (NW1a)knot is located 1.95 west, 1.9 north of the central star. Takinginto account (1) the size of the optical knot (about 0.7 alongthe nebular axis), (2) the sizes of the CXO PSF and the ACIS-S3 pixel size (about 0.5), and (3) the uncertainty in the CXOabsolute astrometry (∼0.6), we believe that the X-ray emissionand the optical knot are cospatial and thus physically associated.The probability of chance association of a background X-raysource with the knot is very small, considering that the opticalknot covers only ∼105 of a region of ∼2 arcmin2 around thesource in which there are only a few (less than five) X-raysources.The spectrum of the X-ray–emitting knot is fairly soft, withalmost all the flux confined to the 0.4–1.6 keV energy band(a total of counts, corresponding to a background-63 8subtracted count rate of counts s1; Fig. 2). The31.5# 10spectrum was extracted using CIAO tools (ver. 2.3), a 5 pixel6 See http://cxc.harvard.edu/ciao.No. 2, 2003 X-RAY EMISSION FROM He 3-1475 L89(2.5) radius circular source region centered on the X-ray knot,and a somewhat larger circular background region centered∼20 west of the source. The spectrum peaks around 0.85 keV,similar to the ACIS-S spectrum of Mz 3 (Kastner et al. 2003).In contrast, two background sources in the vicinity of He 3-1475 have significantly harder spectra.We have fitted the X-ray spectrum with a model consistingof VMEKAL thermal emission plus “standard interstellarmatter”intervening absorption (Morrison & McCammon 1983); a valueof is assumed for the latter (Riera et al. 1995). AssumingA p 2Vsolar abundances, the values of the plasma temperature kT andthe source flux were varied; the best fit, assuming a 20% un-certainty in , gives a temperature, K,6A T p (4.3–5.7)# 10V Xand an “unabsorbed” X-ray flux, 14F p (0.87–1.8)# 10Xergs cm2 s1, where the lower (upper) limit for FX correspondsto the upper (lower) limit for .TXAlthough the above model provides a reasonable fit to theobserved spectrum, the latter shows an excess (relative to themodel) around 0.65 keV. Since oxygen (O viii) can contributeto this feature via line emission at 19 , we have tried modelsA˚in which the O abundance is allowed to vary freely. We findthat there is a hint of O enhancement in the X-ray–emittinggas, but the signal-to-noise ratio is insufficient to determineabundances. Unless, e.g., Fe is severely depleted in He 3-1475(as may be the case for BD 303639; Maness et al. 2003),the best-fit value of obtained from the solar abundance modelTXis likely to be reasonably accurate. The X-ray flux (FX) deter-mination is fairly robust even if heavy metals are severelydepleted.Our derived FX gives an intrinsic X-ray luminosity for He3-1475 of (D/5 kpc)2 ergs s1. Using31L p (4 1.4)# 10Xour model volume emission measure, EM ∼ 1054 cm3, and theelectron density for the northwest knot (∼3000 cm3) fromRiera et al. (1995), we find that the X-ray–emitting volume iscm3, which is a small fraction of the optically emit-47V ∼ 10Xting volume ( cm3), as derived from the knot48V ∼ 3# 10optsize (∼0.2) in the [O iii] image.It is not surprising that X-ray emission is detected only fromthe NW1a knot, since this feature is also the brightest andhighest-excitation optical jet feature, by far, as seen in the[O iii] l5007 image (Fig. 1). For example, the integrated[O iii] flux and the [O iii]/[N ii] emission-line ratio of knotNW1a are larger by factors of 3.5 and 2, respectively, thanthe corresponding values for its point-symmetric counterpart(knot SE1a). The 1.4 mag of differential extinction between theknots ( for SE1 knots; Riera et al. 1995) reduces theA p 3.4VX-ray flux of the SE1a knot by a factor of 6.5 over the NW1aknot, insufficient to make it undetectable in our data. Thus, thedifference in the X-ray emission (and optical emission) betweenknot NW1a and its southeast counterpart probably results froma combination of differences in extinction and excitation.We do not detect a compact X-ray source toward the centralstar in He 3-1475; a conservative upper limit (3 j) on the“unabsorbed” X-ray flux and intrinsic X-ray luminosity, re-spectively, of such a source is ergs cm2 s114F ∼ 1.7# 10cand (D/5 kpc)2 ergs s1, using a temperature31L ∼ 5# 10csimilar to that of the nebular source, and , the valueA p 3.4Vused by Riera et al. (1995) for the central source in He 3-1475.Since the extinction toward the center could be higher and thedistance to He 3-1475 is rather uncertain, our upper limit isnot inconsistent with the value predicted by Blackman et al.(2001a) (∼1032 ergs s1) based on their solar-dynamo analogfor producing magnetic fields in the central stars of pre-PNs.3. DISCUSSIONRecent HST studies of the morphology of young PNs andpre-PNs strongly support a model in which fast collimatedoutflows begin to modify the circumstellar environment duringthe pre-PN phase and/or even earlier, during the very late AGBphase (Sahai & Trauger 1998). Our discovery of nebular 0.4–1.6 keV X-ray emission from He 3-1475 dramatically confirmsthe operation of the resulting violent post-AGB wind/AGBwind interaction process. The simultaneous presence of bothX-ray and optical emission in the NW1a knot supports the ideathat the knots are strongly cooling regions of shocked gas withvery strong temperature gradients. The S-shaped distributionof the fast-moving knots in this object indicates that the fastpost-AGB wind consists of an episodic jetlike outflow whoseaxis has precessed with time (precession angle less than 10and a period ∼1500 yr; Riera et al. 1995). This scenario issupported by the theoretical result that a jet with a quasi-periodic variability and slow precession fragments into clumpsthat behave like “interstellar bullets” (Raga & Biro 1993).Alternatively, the jet may intrinsically consist of a series ofhypersonic bullets ejected by processes close to the surface ofthe star. Matt, Blackman, & Frank (2003) have shown how thedevelopment of toroidal fields in a recently exposed pre-PNcore could lead to an impulsive deposition of flow energy intoa collimated system. Whatever their origin, recent work byPoludnenko, Frank, & Mitran (2003) has shown that hypersonicbullets are capable of recovering key aspects of many pre-PNflows. The temperatures deduced from the X-ray measurementscould be achieved with shock velocities at the surface of thebullet of p 470 km s1. Note that al-1/2v ≈ [(16kT /3m )]X hsthough this speed is lower than that derived for the knots fromobservations (Borkowski & Harrington 2001; Riera et al.2003), the relevant speed for comparison is that of the shocktransmitted into the high-density bullet, which will be lowerthan the bow shock speed. It is also possible that the bulletresponsible for the X-ray knot will be moving into a mediumthat itself has been set into motion by the preceding bullet,leading to lower shock speeds.If, however, the jet is a continuous flow, then the opticalshape of the X-ray knot and its coincidence with the X-rayemission leads to another interesting possibility. The narrowingof the flow at the knot in He 3-1475 is not seen in other jet-producing environments and has been noted as a potential ex-ample of a “conical converging flow” in which material isinertially focused by the walls of an axisymmetric cavity toconverge at a point where shocks redirect the material into ajet (Canto, Tenorio-Tagle, & Ro´z˙yczka 1988; Frank, Balick, &Livio 1996). Recent laboratory experiments have shown suchflows to be capable of producing remarkably stable jets (Le-bedev et al. 2002). While the S-shaped symmetry evident inHe 3-1475 makes pure hydrodynamic collimation unlikely, thepresence of toroidal magnetic fields may produce conicallyconverging flows in a similar manner that would also show X-rays at the convergence point. In fact, He 3-1475 is the firstcase of unambiguously shock-excited X-ray–emitting gas to bedetected by CXO that is unresolved (e.g., all of the PNs detectedby CXO show resolved nebular X-ray emission). The variousjet models for He 3-1475 described above need to be testedusing detailed (magneto)hydrodynamic simulations to quanti-tatively fit the observed X-ray and optical (line) fluxes andspectra.L90 SAHAI ET AL. Vol. 599We are thankful for partial financial support for this workprovided by NASA through its Long Term Space Astrophysicsprogram (grant 399-30-61-00-00) for R. S. and M. M., as wellas through Chandra Awards GO2-3029X (for R. S.) and GO2-3009X (for J. H. K./Rochester Institute of Technology) issuedby the Chandra X-Ray Observatory Center, which is operatedby the Smithsonian Astrophysical Observatory on behalf ofNASA under contract NAS8-39073.REFERENCESBalick, B., & Frank, A. 2002, ARA&A, 40, 439Blackman, E. G., Frank, A., Markiel, J. A., Thomas, J. H., & Van Horn,H. M. 2001a, Nature, 409, 485Blackman, E. G., Frank, A., & Welch, C. 2001b, ApJ, 546, 288Borkowski, K. J., & Harrington, J. P. 2001, ApJ, 550, 778Canto, J., Tenorio-Tagle, G., & Ro´z˙yczka, M. 1988, A&A, 192, 287Chu, Y., Guerrero, M. A., Gruendl, R. A., Williams, R. M., & Kaler, J. B.2001, ApJ, 553, L69Frank, A., Balick, B., & Livio, M. 1996, ApJ, 471, L53Garcia-Segura, G. 1997, ApJ, 489, L189Gardiner, T. A., & Frank, A. 2001, ApJ, 557, 250Guerrero, M. A., Chu, Y.-H., & Gruendl, R. A. 2000, ApJS, 129, 295Guerrero, M. A., Gruendl, R. A., & Chu, Y.-H. 2002, A&A, 387, L1Kastner, J. H., Balick, B., Blackman, E. G., Frank, A., Soker, N., Vrtı´lek,S. D., & Li, J. 2003, ApJ, 591, L37Kastner, J. H., Soker, N., Vrtilek, S. D., & Dgani, R. 2000, ApJ, 545, L57Kastner, J. H., Vrtilek, S. D., & Soker, N. 2001, ApJ, 550, L189Kreysing, H. C., Diesch, C., Zweigle, J., Staubert, R., & Grewing, M. 1992,A&A, 264, 623Lebedev, S. V., et al. 2002, ApJ, 564, 113Li, J., Kastner, J. H., Prigozhin, G. Y., & Schulz, N. S. 2003, ApJ, 590, 586Maness, H. L., Vrtilek, S. D., Kastner, J. 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X-Ray Emission from the Pre-planetary Nebula Henize 3-1475

RIT Scholar Works

Raghvendra Sahai

Joel H. Kastner

Adam Frank

Mark Morris

Eric G. Blackman

MUCC (Crossref)

We report the first detection of X-ray emission in a pre-planetary nebula, He 3-1475. Pre-planetary nebulae are rare objects in the short transition stage between the asymptotic giant branch (AGB) and planetary nebula evolutionary phases, and He 3-1475, characterized by a remarkable S-shaped chain of optical knots, is one of the most noteworthy members of this class. Observations with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory show the presence of compact emission coincident with the brightest optical knot in this bipolar object, which is displaced from the central star by 2'.7 along the polar axis. Model fits to the X-ray spectrum indicate an X-ray temperature and luminosity, respectively, of (4.3-5.7) x 10(exp 6) K and (4 +/- 1.4) x 10(exp 31) (D/5 kpc)(exp 2) ergs s(exp -1) respectively. Our 3 sigma upper limit on the luminosity of compact X-ray emission from the central star in He 3-1475 is approximately equal to 5 x 10(exp 31) (D/5 kpc)(exp 2) ergs s(exp -1). The detection of X-rays in He 3-1475 is consistent with models in which fast collimated post-AGB outflows are crucial to the shaping of nebulae; we discuss such models in the context of our observations

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X-ray Emission from the Pre-planetary Nebula Henize 3-1475

X-Ray Emission from the Pre-Planetary Nebula Henize 3-1475

X-Ray Emission from the Pre-Planetary Nebula Henize 3-1475

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